I have a VisionTek GeForce4 MX 400 card with two heads.
I just can't seem to get it's other head working. Is this a driver limitation, or am I jst doing something wrong. I am currently using a dual head setup with the first head on the card as my first, and a VooDoo 3 3000 as my second. At least this shows that I can get X to use a dual head setup with Xinerama.
I want to really just use my one dual head card though.
Has anyone had any luck getting a dual head nVidia card working? Or more specifically, the GeForce4 MX400.
Any help would be greatly appreciated.

Read the driver docs on how to setup "Twinview". Setting it up works a lot different then setting up normal xinerama. Nvidia uses their own way for it. The driver dir (/usr/share/doc/NVIDIA_GLX-1.0) also contains an example XF86Config which you can use as an example.